So I have a Challenge lab where all I have is 0.1 M NaOH Phenolphthalein Solutio

Question

So I have a Challenge lab where all I have is

0.1 M NaOH

Phenolphthalein Solution

PH Paper

Buret with clamp and stand

pH meter

An Unknown Weak acid.

And I have to find the pKA of the weak acid. Now I know this is a buffer, and that I have to use the henderson hasselbalch equation to determine the pKa, but I don't know how to do this. I can easily test the pH of the solution at any time, but how do I determine the Concentrations of [A-] and [HA]? I can determine equivalence point with the titration, so I will known at what concentration of OH will neutralize the [HA] but I wont know [A-]. Do I have to use half-equivalence point? Does that work for Buffers?

Thanks!

Explanation / Answer

Well. it is very simple to calculate the pKa.

Yes. you need to use the half equivalence point. At half equivalence point pH = pKa of the acid since [conjugate base] = [acid].

So first complete the equivalence point and back calculate the half equivalence point and at that point pH =pKa of acid.

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